The present invention relates generally to primary lithium batteries, particularly thermal batteries; and more particularly to thermal batteries with higher energy densities, that are lightweight, have higher cell voltages, flatter discharge voltages and operate at lower temperatures than presently available thermal batteries.
Applications that require extremely long shelf-life and a burst of power from milliseconds to a few hours use thermal batteries. Thermal batteries are critical to military aviation, equipment, and weapons systems. Applications include aircrew safety systems, air-to-air missiles, air-to-surface missiles, surface-to-surface missiles, surface-to-air missiles and bombs. These systems require the battery to perform reliably under stringent environmental conditions. The less the weight of the battery, the more working payload or the less propulsion lift requirement; thus, every ounce by which the battery weight is reduced means a concomitant increase in agility and acceleration of the missile. Power per unit weight is the crucial figure of merit, with power per unit volume ruing a close second.
Thermal batteries are non-rechargeable power sources, which use electrolytes of inorganic salts that are solid and considered non-conducting at ambient temperatures. Upon ignition of an internal pyrotechnic heat source, the electrolyte melts and becomes conductive, thereby providing power to an external load. Historically, a large number of military systems have utilized thermal batteries. Today, the accepted industry standard is the lithium or the lithium alloy anode based on Li—Si of which over 1.5 million units have been deployed since 1972. Lithium anode based thermal batteries provide high capacity and capability to withstand high dynamic environments.
Advances in thermal batteries have not been running parallel to advances in consumer electronics or OEM batteries. In fact, today's thermal batteries still use the same operating temperatures as they did twenty years ago and the same type of solid electrolytes; therefore the components for the active electrodes are limited in their choice and performance. Those limitations are attributable to the stringent specifications required of the components of the battery. For instance, the Lithium chloride-potassium chloride (LiCl—KCl) eutectic electrolyte (typically with magnesium oxide (MgO) powder binder) in a thermal battery melts at about 352° C., thereby necessitating a significantly higher decomposition temperature for the electrodes than the eutectic temperature of such electrolyte used in the thermal battery. The melting point of the electrolyte determines the effective operating window for its use in a thermal battery. Because of the high operating temperature of thermal batteries (e.g., 400-600° C.), the cathodes for such batteries must be very high temperature stable materials. Furthermore, these cathodes must be electrochemically and chemically stable with the electrolyte. Unfortunately, very few cathode materials meet these criteria.
The most common cathode materials used for thermal batteries are based on the sulfides of iron and cobalt. FeS2 and CoS2 have decomposition temperatures and voltages of 550° C. and 650° C. and 1.94 V and 1.84 V, respectively. vs. lithium. The lithium-silicon alloy anode has a decomposition temperature of about 702° C. As a result, significant thermal management is required for this system to contain all the heat during the battery operation, significantly reducing the energy density of the battery. The low voltage combined with a low capacity and high temperature requirement leads to poor energy density of between 50-80 Wh/kg.
A two-hour thermal battery requires the use of a molten salt that has a lower melting point and larger liquidus range than the LiCl—KCl eutectic, such as lithium chloride-lithium bromide-potassium bromide (LiCl—LiBr—KBr) eutectic, which melts at 321° C. and has a reasonable liquidus range. Another eutectic that has an even larger liquidus range is lithium bromide-potassium bromide-lithium fluoride (LiBr—KBr—LiF), which melts at 280° C.
Several advanced military applications require thermal batteries capable of providing continuous as well as high power pulsed discharges over extended time periods. This need for operational lives in excess of one hour has necessitated an increase of the heat input to the battery for higher starting temperatures. This allows the electrolyte to remain molten over longer time periods. The higher starting temperature requires active materials that are thermally stable at temperatures close to 600° C. The effect of this evolution not only impacts the active materials, but places increasing importance on overall battery thermal management. Sensors placed inside a nose cone of a missile with the thermal batteries on the outside are more prone to failures at these higher temperatures. More thermal insulations are required to protect these sensors, which in turn leads to heavier missiles. Smaller battery packages, for example, will contain smaller cell stack thermal masses and thinner stack insulations. This puts a considerable strain on the performance of the various components in the cell as well as lowering the. energy density due to the extra insulative packaging required for thermal control.
Currently, most missiles incorporate thermal batteries based on the conventional FeS2 cathode, while some incorporate thermal batteries based on an advanced CoS2 cathode. The advanced systems are pushing the limits of current technology in terms of higher power and energy and longer run times in smaller and lighter packages. However, the result is only an incremental improvement over the FeS2 system. Typical tactical and advanced tactical battery applications are showing a trend towards higher battery voltages, low-to-moderate base discharge rates with high pulse loads, relatively small battery envelopes, and substantially increasing mission times. Conversely, strategic battery requirements tend to require longer mission lives, higher current requirements with steady and/or pulsing loads, larger battery envelopes due to higher power requirements, and may involve maximum skin temperature specifications primarily due to longer mission lives.
A very large number of oxides, because of their refractory properties, have been explored for use as the cathode material, but none to date is believed to have provided a viable system that is highly conductive and thermally stable at the operating temperatures required of these batteries. The materials considered include oxides based on titanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu). Although desirable attributes were found in many of these oxides, such as high voltages, high energy densities, and good thermal stabilities, many disadvantages were also found, including sloping voltage discharge, inability to utilize full capacity, and oxidation of the halogen-based electrolyte to free halogen by the high voltages.
Some recent reported developments include thin film technologies involving plasma spraying of cathodes and electrolyte components, or tape casting and consolidating all cell layers. Although very high power can be achieved from such designs, the problems of lower energy density and thermal insulation remain, and cost is expected to be higher because of the exotic deposition technique.
As noted above, present thermal batteries operate at very high temperatures (400° C.-600° C.), which is one of their disadvantages. Most of the electrolytes developed so far are based on the halogen derivatives such as LiCl—KCl or LiF—LiCl—LiBr eutectic mixtures or variations of them. However, almost all the electrolytes presently in use or previously proposed for use operate at very high temperatures, thus requiring a cathode with a higher decomposition temperature. Improvements could be made in the energy density and performance characteristics of thermal batteries if a cathode material were found with properties of high capacity per unit weight and volume, thermal stability at high temperatures, high voltage output, very high electronic conductivity, very high thermal conductivity, high reaction kinetics, wide electrochemical stability window, flat voltage with discharge, and, above all, lack of reaction with or oxidation of the electrolyte. The latter is a very important feature that has precluded the use of high voltage cathodes since the common electrolytes are based on the halogen salts, which tend to oxidize to free halogen gases. A wide range of battery chemistries exists today but only a handful may be suitable for use in the development of advanced thermal batteries. Most of the problems are associated with decomposition of the components at the thermal battery temperatures, thermal conductivity, electrochemical instability, or sloping cell voltages.
Clearly, major improvements are needed to reduce the present weight and volume of these batteries in such applications. Future thermal-battery applications envision higher energy densities and lifetimes of up to four hours. The current technology does not meet these requirements, primarily due to limitations of the cathode material.